It has long been recognised in the art that non-variable valve duration in a four cycle internal combustion engine is a serious impediment to optimal engine efficiency, and in view of this deficiency many systems have been proposed to provide continually variable valve duration.
One class of system proposed has been to employ various mechanisms in which the opening flank of a first camlobe opens the valve, while the closing flank of a second camlobe controls the closing of the valve, and by variably indexing the first and second camlobes relative to each other, achieving a variation in the duration of the valve.
A serious problem with this approach is with regard to the reconciliation of valve motion at the point in the valve lift curve where the opening and closing lobe flanks effectively meet. At any worthwhile extension of duration, a dynamically unacceptable unevenness in the curve of valve lift develops, as depicted in FIG. 6.
A solution often proposed to solve this problem has been to employ a pair of camlobes with broadly flattened "tips" defined by a radius rotated about the cam axis, thereby allowing a continuous transition from one lobe to the other at more extended durations.
The benefits of this approach are largely illusory, since a camlobe with such a broad "tip" has, of necessity, a very long duration, rendering the minimum duration the system can transmit to the valve excessive; or, if the duration of the lobe is usefully short, pushing valve accelerations beyond any acceptable levels.
A further serious limiting factor in camlobe design which prevents realisation of optimal valve action, both in systems which seek to vary valve duration, and in non-variable designs, is the necessity, in camlobe design, to limit valve accelerations (both positive and negative) to those that will be developed at the highest r.p.m.'s the engine will attain in use. Unfortunately, the use of a camlobe profile that will develop maximum allowable valve accelerations at high r.p.m. will result in less than optimum valve opening and closing rates at all engine speeds below that maximum. In short, the rate of valve opening and closing should ideally increase progressively as engine speed drops, this being possible by virtue of the increasingly longer time available to open and close the valve as engine speed decreases. The result of such an ideal state of affairs would be to substantially increase cylinder filling at all engine speeds; the higher volumetric efficiency resulting producing a much improved torque curve, and superior power characteristics. Perhaps more importantly, at the present time, is the fact that the realisation of both of the above factors of fully variable valve duration, and variable rates of valve opening and closing, would offer a predictable baseline of engine induction and exhaust characteristics upon which to base development of fuel economy and emission control factors.
Accordingly, a mechanism is proposed to provide a means, when used in conjunction with any suitable system employing two camlobes with variable phasings between them to actuate a valve at variable durations, to reconcile the motions of said camlobes; and to vary, towards an optimum value, rates of valve opening and closing according to engine speed.